Corrosion Theory

CORROSION THEORY DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 2 Corrosion of the nuclear fuel cladding may cause the cladding to become brittle and less ductile. The swelling from the generation of fission gases within the fuel may then cause the cladding to crack or blister, and highly radioactive fission products may then be released to the coolant. Some of the metallic oxide corrosion products released to the coolant may be deposited on surfaces in the reactor core. The neutron flux in the core produces nuclear reactions in the corrosion products, and they become highly radioactive. Subsequently, these activated corrosion products may be released from the core and redeposited on surfaces outside the core region. The radiation fields resulting from this redeposited material may then significantly increase radiation levels, thereby complicating maintenance and access capabilities. The corrosion product oxides may also result in fouling of heat transfer surfaces and in the accelerated wear of moving parts by corrosion products trapped in or between them. Current is the flow of electrons through a medium. An electric current can flow through a metal conductor, and the metal will not show any obvious chemical changes. This type of conduction of electricity is called metallic conduction. Ionization is the process of adding electrons to or removing electrons from atoms or molecules, creating ions. High temperatures, electrical discharges, and nuclear radiation can cause ionization. Many metals have a tendency to lose electrons to atoms or ions that have a tendency to gain electrons. Current can be conducted by the movement of these ions. The compounds that conduct electric current by ion movement are called electrolytes, and this ionic motion is call electrolytic conduction. Conductivity is a measure of the ability of a substance to allow electron flow. In the context of corrosion, conductivity indicates the amount of ions in solution, which relates directly to the potential of corrosion taking place. Corrosion is the deterioration of a material due to interaction with its environment. Corrosion can have many forms, both wet and dry. Electrolysis is the decomposition by electric current (in the context of corrosion the use of electrical current to bring about chemical change). This chapter will concentrate on the corrosion of metals in water-based environments (electrolytes). An electrolyte is defined as an electricity-conducting fluid; that is, it has positive and negative ions that can move and constitute an electrical current. Pure water has a limited number of dissociated H and OH ions and is a relatively poor conductor of electricity. Addition of acids, + - bases, or salts that dissociate into ions increases the current-carrying capability of the water (electrolyte).